The use of inorganic oxides such as silica or metal halides such as magnesium chloride as inert supports for olefin polymerization catalysts is well known in the art. Reports in which silica serves as an additional support for magnesium chloride based Zielger-Natta polymerization catalysts are also available (see for example U.S. Pat. Nos. 5,798,314, 5,661,097, and 6,642,325). However, the use of magnesium chloride treated silica supports for the immobilization of single site catalysts is relatively rare.
U.S. Pat. Appl. No. 2009/0163682 A1 discloses a supported “metallocene” catalyst which catalyzed slurry phase ethylene polymerization in the absence of traditional co-catalysts or activators such as methylaluminoxane. The “metallocene” catalyst has a single cyclopentadienyl type ligand and three halide ligands and is immobilized on a silica support which has been treated with an organoaluminum species and magnesium chloride. Preparation of the support employs magnesium chloride directly, and hence requires the use of a polar solvent for the solubilization of the magnesium chloride. The polyethylene products obtained had very high molecular weights and broad molecular weight distributions. Overall catalyst activities were fair and productivity reached more than 1000 gram of polymer per gram of supported catalyst. The use of a mono-cyclopentadienyl titanium trichloride species in the catalyst formulation may be more consistent with the formation of a traditional Ziegler-Natta multisite catalyst, as evidenced by the broad molecular weight distributions observed for the polymer products.
Indeed, a similar preparation reported in U.S. Pat. No. 5,324,698 employs a mono-cyclopentadienyl titanium trihalide catalyst to prepare a Ziegler-Natta catalyst on a silica support. That a multi-site catalyst was formed was supported by the high molecular weight distributions obtained for the product polymers (e.g. MW/Mn of 3.95 and higher) Again, magnesium chloride was used directly as a solution in tetrahydrofuran (along with mono-cyclopentadienyl titanium trichloride) for addition to a silica support.
U.S. Pat. No. 7,402,546 discloses a support for an olefin polymerization catalyst which comprises an inorganic oxide, such as silica, coated with magnesium chloride. Metallocenes, non-metallocene single site catalysts and Ziegler-Natta catalysts are contemplated for immobilization on the modified supports. Formation of the coated supports involved dissolving magnesium chloride in a polar solvent (e.g. ethanol), followed by addition to silica, resulting in magnesium chloride like particles having uniform and small particle size.
U.S. Pat. No. 6,187,882 describes the use of a mixed inorganic oxide/magnesium halide support for metallocene catalysts. The catalysts further comprise an electron donor compound such as tetrahydrofuran (THF) which is used to solubilize magnesium chloride during the catalyst preparation. The supported catalyst formulations do not comprise a cocatalyst such as MAO, which is added instead, directly to the polymerization reactor. The metallocenes employed in the examples are bis-cyclopentadienyl or bis-indenyl catalysts based on zirconium.
The above prior arts employ a strategy in which pre-made magnesium chloride is solubilized in a polar solvent prior to addition to an inorganic oxide support. Polar solvents can have deleterious effects on single site polymerization catalyst performance due to their tendency to coordinate to the active polymerization metal center.
U.S. Pat. Nos. 5,763,349 and 6,214,948 both teach the use of a silica-magnesium chloride mixed support for a metallocene catalyst. The support is formed by tumbling dry silica powder with anhydrous magnesium chloride powder. A metallocene catalyst is added to the support in powder form and in the absence of a liquid. The catalyst formulation is completed by adding a solution of an ionic activator such as triphenylcarbenium tetrakispentafluorophenylborate. Tumbling a dry magnesium chloride powder with a silica powder limits contact between the two substrates and may not be an efficient way to ensure an even distribution of a magnesium chloride within the silica pores.
The present invention overcomes the limitations of the prior art by providing a passivated inorganic oxide support by treatment with magnesium chloride that is generated in situ.